Against a failure occurring in a power system so-called short circuit failure or earth faulting, a protection relay device installed in each element such as a transmission line, transformer, bus, and generator forming a power system operates with respect to a failure occurring within its territory. When a section containing the failure point is separated from the left trouble-free power system through tripping of a breaker, a failure is removed. At present, there goes mainstream a digital protection relay in which current information or voltage information of power system is digitized and algorithm for determining a system failure is realized through software process.
As its function, hopefully, a power protection relay correctly detects a failure within a previously allocated protection object and fast separates from a power system a section which a failure occurrence section can be minimized. Accordingly, ideally, a device referred to as a main protection device which ought to operate at first at the time when a failure occurs fast operates if at all possible in an internal failure and fails to operate in an external failure. Accordingly, as a method for accurately detecting a failure, a current differential principle is widely used for a relay for the main protection device.
However, as in a transmission line protection device, current information of both ends of transmission lines is necessary for using a current differential principle. In this case, a relay is installed in both ends of the transmission line and both the ends are connected by a communication line. At least at one end, the current information units of both the ends are collected, thereby using the current differential principle. Through congestion of transmission lines, multiple-terminal transmission lines are currently used and transmission lines having five or six terminals are more used.
As can be seen from the above sequence, current information in which sampling synchronization of all terminals is accomplished and communication paths for collecting the current information are necessary for applying a protection relay of the current differential principle to a multiple-terminal transmission line. As a method for configuring a communication path in the case where a multiple-terminal transmission line is used, there are used a representative terminal judgment type in which from a representative terminal, the left terminals are connected through communication lines in a radial pattern and data of all terminals is collected to the representative terminal, and a loop system in which a relay is installed in each terminal, a terminal is connected to each other in a loop form through a communication line, and data of all terminals is collected (see, for example, NON PATENT LITERATURE 1).
For realizing a protection of a high-accuracy transmission line, high-precision sampling synchronization is needed. A sampling synchronization method in which a delay in a transmission path is considered is each applied to a configuration of the communication path (see, for example, NON PATENT LITERATURE 1).